Pick > Pick Reference

Picks and unpicks individual objects, alternatives, or layers of a reference file, or the whole reference file, in the view window.

Pick Reference Options

Pick Level: Specifies at what level your are picking within the reference file. Choose between: Component, Layer, File, or Alternative.
Curves/Surfaces/Shells/Meshes: If Pick Level is set to Component, you can further refine what type of objects can be picked by checking or unchecking these options.

By default, they are all checked.